`<xloctime>`: `time_put::do_put` does not match `strftime` for the `%c` and `%r` specifiers in the "C" locale

[cpplearner]

Describe the bug

The C standard says strftime's %Ec and %c are equivalent to %a %b %e %T %Y and %r is equivalent to %I:%M:%S %p in the "C" locale. UCRT's strftime does something special to meet this requirement, but this magic doesn't extend to time_put::do_put.

[locale.time.put.virtuals]/1 is extremely vague regarding locale-dependent formatting:

the sequence of characters produced for those specifiers that are described as depending on the C locale are instead implementation-defined.

The libcxx test std/localization/locale.categories/category.time/locale.time.put/locale.time.put.members/put2.pass.cpp expects do_put to match strftime for these specifiers.

Command-line test case

D:\test>type test-time-put.cpp

#include <cassert>
#include <ctime>
#include <iostream>
#include <locale>

class my_facet : public std::time_put<char, char*>
{
public:
    explicit my_facet(std::size_t refs = 0)
        : time_put(refs) {}
};

int main() {
    const my_facet f(1);
    std::tm t = {};
    t.tm_sec = 6;
    t.tm_min = 3;
    t.tm_hour = 13;
    t.tm_mday = 2;
    t.tm_mon = 4;
    t.tm_year = 109;
    t.tm_wday = 6;
    t.tm_yday = 121;
    t.tm_isdst = 1;
    std::ios str(nullptr);

    {
        char buf[200];
        char* iter = f.put(buf, str, '*', &t, 'c');
        std::string ex(buf, iter);
        std::cout << ex << '\n';
    }
    {
        char buf[200];
        strftime(buf, 200, "%c", &t);
        std::cout << buf << '\n';
    }
    {
        char buf[200];
        char* iter = f.put(buf, str, '*', &t, 'r');
        std::string ex(buf, iter);
        std::cout << ex << '\n';
    }
    {
        char buf[200];
        strftime(buf, 200, "%r", &t);
        std::cout << buf << '\n';
    }
}

D:\test>cl /EHsc test-time-put.cpp

Microsoft (R) C/C++ Optimizing Compiler Version 19.50.35725 for x64
Copyright (C) Microsoft Corporation.  All rights reserved.

test-time-put.cpp
Microsoft (R) Incremental Linker Version 14.50.35725.0
Copyright (C) Microsoft Corporation.  All rights reserved.

/out:test-time-put.exe
test-time-put.obj

D:\test>test-time-put.exe

05/02/09 13:03:06
Sat May  2 13:03:06 2009
13:03:06
01:03:06 PM

Expected behavior

do_put has the same output as strftime.

STL version

117ca96

Additional context